1. Field
The disclosed concept pertains generally to direct current (DC) power systems and, more particularly, to protection systems for feeders between combiner boxes and inverters.
2. Background Information
Solar and other renewable energy solutions involve power production at a direct current (DC) level and conversion to alternating current (AC) for usage with traditional electrical load devices. As one example, due to the distribution of DC power production from many solar panels, these many sources of DC power are combined via electrical conductors and other electrical connections. These many sources are combined and there is potential for back feeding from adjoining power sources into DC faults that may result from, for example, faulty construction, wear and tear, and environmental stresses.
The National Electric Code provides guidelines to prevent such back feeding via the use of fuses or circuit breakers at the connection point of each string of solar panels into a combiner box. The power that is combined within the combiner box is then distributed to a DC-to-AC inverter that converts the DC power to AC power. This connection between the combiner box and the inverter has limited known protection schemes available and/or in use. One known system design allows for relatively small DC faults within a string of solar panels to exist without interruption by an electrical protective device. These relatively small faults may not progress as in traditional faults since the source of energy, the sun, disappears in the evening, therefore allowing overheated insulation to cool, and then re-heats during the next day. This cycling affect of the energy source, the sun, does not allow for the relatively fast transition from a small fault into a larger fault that would be interrupted by an electrical protective device. Should these string faults grow, they would be back-fed from other photovoltaic (PV) strings and the required fuse would open and therefore isolate one leg of the faulted string. However, known designs do not provide any protection for the second leg of the faulted string.
When an existing low level fault exists on one of the two legs within the string and a second fault occurs between the combiner box and the inverter, there is potential for an ongoing fault condition which will not be interrupted by known electrical protective devices. In addition, known solar system designs do not have any mechanism to isolate the source of DC power generation, as long as the sun is shining. A known method to terminate the energy generation would be to “cover” the solar panels from the sun light, which is not practical. Other possibilities include DC/DC converters and rooftop disconnects which have a remote shutdown feature, although for relatively larger PV systems the cost becomes excessive.
Protection of PV arrays presents unique challenges that stem from the limited output nature of photovoltaic sources (i.e., their output current into a short circuit is only slightly higher than full load current). Furthermore, since the output under both load and short circuit is directly related to irradiance, under less than full sun the output into a fault may be significantly less than normal load conditions. Considering that all overcurrent devices must be sized based on maximum short circuit current, it quickly becomes evident that conventional time-overcurrent protection (e.g., fuses; conventional circuit breakers) is insufficient, and a more advanced method of fault detection is needed.
Arc fault circuit interruption (AFCI) protection was recently accepted for NEC 2011.
Ground fault circuit interruption (GFCI) protection is also required by the NEC, but is typically implemented only at the inverter at one end and one location of the DC feeder.
There is room for improvement in DC feeder protection systems.